1. Field of the Invention
The invention relates to a misfire determination device and misfire determination method for an internal combustion engine. Particularly, the invention relates to measures for improving reliability of determination in a misfire determination device and misfire determination method that determine whether a misfire has occurred on the basis of a rotation fluctuation of an internal combustion engine.
2. Description of the Related Art
So far, for example, as a so-called “misfire” occurs, an unburned air-fuel mixture is exhausted to an exhaust passage. The misfire is a phenomenon that an air-fuel mixture fails to be ignited in an internal combustion engine for an automobile. The misfire may deteriorate exhaust emission or adversely influences an exhaust gas purification catalyst.
Then, a misfire determination device has been proposed. The misfire determination device focuses on that a fluctuation in engine rotational speed (hereinafter, simply referred to as rotation fluctuation) increases when the misfire occurs. The misfire determination device determines, on the basis of the rotation fluctuation, whether a misfire has occurred. The basic principle of misfire determination in the device of this type is as follows.
First, when a misfire occurs in one cylinder, an engine rotational speed in the expansion stroke of that cylinder gradually decreases. As a result, a duration required for a crankshaft to rotate a certain crank angle in the expansion stroke of the misfire cylinder is longer than a duration required for the crankshaft to rotate the certain crank angle in the expansion stroke of another cylinder. Thus, by measuring and comparing these periods of time, it is possible to determine whether a misfire has occurred.
Specifically, when a cylinder (for example, a third cylinder) is in the expansion stroke, a difference between a duration required for the crankshaft to rotate the certain crank angle in the expansion stroke of the third cylinder and a duration required for the crankshaft to rotate the certain crank angle in the expansion stroke of a cylinder (for example, a second cylinder) that is in the expansion stroke a predetermined crank angle before (for example, 360° before) the above expansion stroke is calculated (the latter duration is subtracted from the former duration). Then, when the calculated value exceeds a predetermined threshold, it is determined that the rotation fluctuation of the internal combustion engine has increased and then it is determined that a misfire occurs (misfire occurs in the third cylinder).
An automobile intended for countries that employ an on-board diagnostic system (OBD) is equipped with a misfire counter in a control circuit. The misfire counter is incremented each time a misfire determination is made. When the count value of the misfire counter per predetermined number of revolutions of the engine (for example, 1000 revolutions) exceeds a predetermined value (for example, 30), an MIL (warning lamp) lights up to warn a driver.
Thus, under a situation that a misfire determination is not accurately made, for example, a misfire may not be recognized despite occurrence of a misfire, so deterioration of exhaust emission continues. In addition, occurrence of a misfire may be erroneously determined despite no occurrence of a misfire, and the MIL early lights up even when an actual number of misfire occurrences is smaller than the predetermined value. This makes a user feel uncomfortable.
Then, to eliminate an erroneous misfire determination, there is known a technique in which it is determined that a misfire occurs when the temporal variation pattern of a rotation fluctuation coincides with a predetermined misfire pattern (for example, see the following Japanese Patent Application Publication No. 2006-152971 (JP-A-2006-152971)).
The technique described in JP-A-2006-152971 detects a duration required for a crankshaft to rotate a certain crank angle in an expansion stroke of each cylinder, calculates a difference in the duration between cylinders of which expansion strokes are sequentially carried out with an intervening expansion stroke (between cylinders of which expansion strokes are sequentially carried out at an interval of 360° crank angle in the case of a four-cylinder engine), and then determines that a misfire occurs when the calculated difference exceeds a threshold and the variation pattern of the difference coincides with a pattern specific to occurrence of a misfire. Specifically, when the rotation fluctuation amount in the expansion stroke of a misfire determination target cylinder exceeds a predetermined threshold, and the rotation fluctuation amount is particularly larger than the rotation fluctuation amount of a cylinder that is in the expansion stroke immediately before the expansion stroke of the misfire determination target cylinder and the rotation fluctuation amount of a cylinder that is in the expansion stroke immediately after the expansion stroke of the misfire determination target cylinder, it is determined that a misfire occurs in the misfire determination target cylinder.
In addition, Japanese Patent Application Publication No. 4-171249 (JP-A-4-171249), Japanese Patent Application Publication No. 10-331707 (JP-A-10-331707) and Japanese Patent Application Publication No. 2004-293350 (JP-A-2004-293350) describe a technique that, in an automobile that includes a torque converter equipped with a lock-up clutch, a threshold for engine misfire determination is varied between when the lock-up clutch is in a lock-up state and when the lock-up clutch is in a non-lock-up state.
However, the inventors of the invention found that an erroneous misfire determination may still possibly occur in the misfire determination techniques described in the above Patent Publications.
Specifically, in an automobile equipped with an automatic transmission, a torque converter includes a lock-up clutch, and an engine is directly coupled to the automatic transmission when the lock-up clutch is engaged (in a lock-up state). Under the above situation, as a misfire occurs to generate vibrations in the engine, the vibrations are transmitted to the automatic transmission. If the frequency of the vibrations coincide with the specific frequency of the automatic transmission, this causes a resonance phenomenon of an overall power train from the engine to the automatic transmission. Under the situation that the above resonance phenomenon occurs, it is difficult to accurately identify the rotation fluctuation, and there is a possibility that an erroneous misfire determination may be made.
An example of a situation under which an erroneous misfire determination occurs will be described. An engine rotational speed is measured in such a manner that a crank angle sensor formed of an electromagnetic pickup detects a passage of external teeth of an NE rotor that is integrated with the crankshaft. During then, if the resonance phenomenon occurs, a resonance also occurs in a stay that supports the crank angle sensor. This changes a relative position between the crank angle sensor and the NE rotor. For this reason, even when the crankshaft rotates at a certain rotational speed, a passage timing of the external teeth of the NE rotor detected by the crank angle sensor is unstable because of a change in relative position due to the resonance phenomenon. Conversely, although the rotational speed of the crankshaft fluctuates because of a misfire, the rotation fluctuation may not be accurately detected because of the resonance phenomenon. As described above, the resonance phenomenon causes amplification, cancellation, attenuation, or the like, of rotation fluctuation, and, therefore, the rotation fluctuation is not accurately detected by the crank angle sensor. This may cause an erroneous misfire determination.
In addition, the above described JP-A-4-171249, JP-A-10-331707 and JP-A-2004-293350 describe that a threshold for misfire determination is changed between when the lock-up clutch is in the lock-up state and when the lock-up clutch is in the non-lock-up state; however, an erroneous misfire determination due to a resonance phenomenon of the overall power train is not considered at all, and a misfire determination is simply made on the basis of whether the rotation fluctuation amount exceeds a predetermined threshold. Thus, there is still a possibility to make an erroneous misfire determination due to the resonance phenomenon in these JP-A-4-171249, JP-A-10-331707 and JP-A-2004-293350.